1. Field of the Invention
This invention relates generally to magnetic recording disk drives, and more particularly to a disk drive that includes a system for canceling the effects of rotational and linear vibration.
2. Description of the Related Art
Magnetic recording hard disk drives use an actuator, typically a rotary voice-coil-motor (VCM) actuator, for positioning the read/write heads on the data tracks of the recording disks. The disk drive has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track and move them to the desired track for reading and writing of data.
Disk drives experience rotational vibration (RV) and disturbance forces during normal operation. These disturbances arise internally, such as from motion of the VCM actuator, as well as externally, such as from shocks to the frame supporting the disk drive or from the movement of other disk drives when the drives are mounted together in a disk array system. RV tends to rotate the disk drive housing about an axis parallel to the axis of disk rotation and thus moves the disks relative to the actuator. RV cancellation is a method that uses sensors to detect RV and improve the PES by canceling the off-track motion induced by the RV. One approach uses two linear accelerometers or vibration sensors, typically mounted on the disk drive's printed circuit board, to measure the RV. The measured RV is input to a feedforward controller that creates a feedforward compensation signal that is summed with the control signal to the VCM actuator. This method is sometimes called RV feedforward (RV-FF). Disk drives with two vibration sensors for RV-FF are described in U.S. Pat. No. 6,961,202 B2 and US 20100067357 A1, both assigned to the same assignee and this application.
However, disk drives are also subject to translational or linear vibration and disturbance forces, which tend to move the disk drive housing along an axis parallel to the disks. Linear vibration in the disk radial or cross-track direction is also major a cause of off-track motion of the head. Linear vibration in the disk circumferential or along-the-track direction is also a cause of off-track motion of the head. U.S. Pat. No. 6,496,320 B1 describes a disk drive with three linear accelerometers that detect linear vibration along orthogonal x and y axes aligned with the width and length of the disk drive housing.
What is needed is a disk drive that detects and compensates not only for RV but also for linear vibrations in both the cross-track and along-the-track directions.